Higher activity of ni/γ-al₂o₃ over fe/γ-al₂o₃ and ru/γ- al₂o₃ for catalytic ammonia synthesis in nonthermal atmospheric-pressure plasma of n₂ and h₂

Developing a novel ammonia synthesis process from N₂ and H₂ is of interest to the catalysis and hydrogen research communities. γ-Alumina-supported nickel was determined capable of serving as an efficient catalyst for ammonia synthesis using nonthermal plasma under atmospheric pressure without heating. The catalytic activity was almost unrelated to the crystal structure and the surface area of the alumina carrier. The activity of Ni/Al₂O₃ was quantitatively compared with that of Fe/Al₂O₃ and Ru/Al₂O₃, which contained active metals for the conventional Haber–Bosch process. The activity sequence was Ni/Al₂O₃ > Al₂O₃ > Fe/Al₂O₃ > no additive > Ru/Al₂O₃, surprisingly indicating that the loading of Fe and Ru decreased the activity of Al₂O₃. The catalytic activity of Ni/Al₂O₃ was dependent on the amount of loaded Ni, the calcination temperature, and the reaction time. XRD, visual, and XPS observations of the catalysts before the plasma reaction indicated the generation of NiO and NiAl₂O₄ on Al₂O₃, the latter of which was generated upon high-temperature calcination. The NiO species was readily reduced to Ni metal in the plasma reaction, whereas the NiAl₂O₄ species was difficult to reduce. The catalytic behavior could be attributed to the production of fine Ni metal particles that served as active sites. The P_N2/P_H2 ratio dependence and rate constants of formation and decomposition of ammonia were finally determined for 5.0 wt % Ni/Al₂O₃ calcined at 773 K. The ammonia yield was 6.3% at an applied voltage of 6.0 kV, a residence time of reactant gases of 0.12 min, and P_H2/P_N2 = 1.